The present invention relates to a drawing surface adjusting mechanism for use with a scanning pattern drawing apparatus such as a laser plotter which describes patterns by scanning with a laser beam. More particularly, the present invention relates to a drawing surface adjusting mechanism that is capable of increasing or decreasing the distance between an optical system and the workpiece by tilting the latter with respect to the former.
Conventional laser plotters are intended to draw patterns that require only a comparatively low precision. In such applications, the diameter of the beam spot which determines minimum line width is on the order of 30 .mu.m. To allow for pattern drawing with a higher precision and a smaller line width, the spot diameter must be further reduced by shortening the focal length of the scanning lens. However, a scanning lens with a shorter focal length unavoidably has a smaller depth of focus, and hence if the workpiece on which a pattern is to be drawn is displaced either upward or downward with respect to the optical system, the entire drawing surface may be located beyond the depth of focus. On the other hand, if the drawing surface is inclined with respect to the optical system, part of the scanning field or subfield may be located beyond the depth of focus.
To overcome these problems, some provision must be made for detecting the amount of displacement or inclination of the workpiece and performing the necessary correction. In practice, however, a large scanning pattern drawing apparatus involves considerable difficulty in focusing at various positions of a single scan stroke whereby vertical movements of the drawing surface must be effected to perform focusing. Hence, it is desired that the position of the drawing surface be held constant for a single stroke of scanning.
Further, in the presence of waviness and other flaws in the drawing surface, satisfactory pattern drawing can of course be accomplished if the difference in the height of the workpiece within the width of a single stroke of scanning is within the depth of focus. Even if the difference in height on the horizontal drawing board is beyond the depth of focus, it can be adjusted to be within the depth of focus by tilting the drawing board.